An intraocular lens has a principal refractive structure, known as a lens optic, and one or more support structures for positioning and centering the lens optic within the anterior or posterior chamber of an eye. Commonly referred to as "haptics", these support structures may be integrally formed with the lens optic (a one-piece lens), or separately manufactured and attached to the lens optic (a multi-piece lens).
An important goal for intraocular lens design is to minimize trauma to the eye when the lens is inserted. To that end, effort is made to ensure, for example, that the incision to the eye is kept small during the implantation operation; that biologically inert materials are used in the construction of the intraocular lens; and that the physical proportions of the lens do not interfere, irritate, or damage delicate inner eye tissue.
What makes achieving the design goals difficult is that often the characteristics necessary for a good lens optic are undesireable for the lens haptic, and vice versa. The two have conflicting design requirements.
Conventional intraocular lens optics, for instance, are commonly made from biocompatible materials such as polymethylmethacrylate (PMMA). With this rather rigid material, lens optics are easily cast or machined into their final form. So in regard to handling ease and manufacturability, the benefits of PMMA are obvious. By the same token, because this material is rigid, many of the foregoing design goals are compromised.
More recently, however, more flexible materials have been devised for the lens optic. Flexible lens optics cast of elastomeric materials such as silicone or hydrogels, for example, have gained popularity because they produce foldable intraocular lenses that may be inserted through a beneficially small incision in the eye.
Once the intraocular lens is implanted, the haptics must hold the lens optic in proper alignment with the optical axis of the eye as well as support the weight of the lens optic. The haptics must therefore be sufficiently rigid to perform their function. In short, haptics must simultaneously be pliant enough to avoid damaging delicate eye tissue yet rigid enough to act as a support structure.
To be sure, the majority of the so called "small incision lenses" have been limited to multi-piece designs. One reason is that a small incision lens connotes a flexible lens that is folded during implantation. Experience has seen that a flexible lens optic material that is desirable for the optic is usually too flimsy to work for the haptic in its support function--hence, the evolution toward the multi-piece lens design.
The type of material is also an issue. Elastomerics commonly used for the optic do not perform satisfactorily as a haptic, except in a broad flange configuration, which is less desirable than other more streamlined configurations. As a result, flexible intraocular lenses depend on more rigid polypropylene monofilament haptics.
A wide variety of haptic configurations intended for use with silicone or other elastomeric lens optics have been produced by permanent deformation of an elongated filament, as disclosed in U.S. Pat. No. 4,880,426 to Ting et al.; or by staking in the lens optic an anchor formed at an end of the filament haptic, as taught in U.S. Pat. No. 4,894,062 to Knight et al. Unfortunately, the Ting and Knight intraocular lenses exhibit only moderately satisfactory pull strengths. As is known in the art, pull strength is a measure of the haptic's ability to resist detachment from the lens optic when subjected to an outward, radial tensile force. Such a force, among others, is commonplace during implantation surgery.
In order to obtain acceptable pull strengths, some filament haptics are provided with an enlarged anchoring head that helps secure it to a flexible lens optic. But an enlarged anchoring head is usually difficult to form consistently because conventional manufacturing techniques involve, for example, winding an end of the monofilament material around a small diameter mandrel and ultrasonically welding the overlapping part of the filament to fix the looped shape. This technique is generally disclosed in U.S. Pat. No. 4,790,846 to Christ et al. The welding is necessary because without it, the loop cannot hold its form. Once the shape collapses, it is easy for the haptic to detach from the lens optic. Additionally, even if the loop were welded closed, the filament may be too flexible to retain the loop shape under tension, and again the loop would collapse.
Although the prior art looped-shape anchoring head helps interlock the haptic to the lens optic, and the design has met with some commercial success, it does have drawbacks. The process steps undergone in creating the looped anchoring head are extremely labor intensive, and require highly-trained technicians to skillfully guide intricate tools while observing through a magnifying lens. As such, it is difficult to maintain consistently high quality in the finished product. Also, because so much labor is involved, high production speeds cannot be attained. Consequently, conventional intraocular lenses of this type are not easily adapted to automated mass production, and production costs are significant.
Furthermore, by wrapping an end of the filament around the mandrel and welding it to create the closed loop, a double thickness of haptic material at the point of overlap is made. This double thickness may be greater than the thickness of the optic itself, causing the haptic to protrude from the lens surface. In the alternative, the looped anchoring head may be positioned closer to the thicker central optical zone of the lens and away from the thinner lens periphery. Unfortunately, the presence of the anchoring head in the optical zone may distort or detract from the image seen through the lens optic.
Another disadvantage inherent in the welded-loop anchoring head haptic is the potential for the weld to break as the filament is subjected to longitudinal stress. This has been known to result in the haptic pulling away from its anchoring point and out of the optic altogether.
Insofar as the weld itself is concerned, it may be prone to chemical degradation, which may contaminate the ambient environment after implantation. Such an occurrence could be catastrophic in the eye because it may lead to vision problems.
There have been attempts at configuring other shapes for the enlarged anchoring head, aside from welded-loop discussed above. For instance, the attempts of Ting, Knight, as well as those seen in U.S. Pat. No. 4,888,013 to Ting et al. and U.S. Pat. No. 4,978,354 to Van Gent collectively disclose enlarged anchoring heads having a triangular shape, a saw-tooth shape, an arrow-head shape, a knob shape, a barbed hook shape, and a hammer-head shape. The resulting haptics, however, have proven inadequate for a variety of reasons, for instance: (1) reliance on bonds that may fail or chemically leach into the environment; (2) non-adherence of optic/haptic materials; (3) an axially symmetrical anchoring head design that cannot resist torque along that rotational axis; or (4) the anchoring head shapes are too bulky.
A key to superior pull strength is the amount of surface area that the anchoring head engages in a specific direction within the lens optic. Indeed, it determines the pull strength, and the ability of the haptic-optic joint to withstand torsional and bending forces.
Therefore, there is a need for an improved haptic that exhibits high pull strength, and resists torsional and bending forces, yet easily adapts to mass production, and is biocompatible.